I'm pretty sure the .243 class of cartridges is not acceptable in the Contender/G2 Contender platform.
As stated before, nearly anything using the .223 head size is acceptable, or the larger, but lower pressured cartridges like the .30-30 and .45-70.
TC lists the 6.8 Rem, but not the 6.5...

Personally, I'd go with either the .204 Ruger, or the .221 Fireball. I think (haven't verified) the Fireball will get almost the same velocities as the .223 out of the 15" barrels, but will use less powder to do so.
Both of those are factory cartridges, so dies shouldn't be a problem.
If you are just wanting something different, I'd look at one of the .17's... like the .17 MachIV, or even the little .17 K-Hornet, or the new factory duplicate... what's it called... the .17 Hornady Hornet? Something like that.

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I always felt that if I got to the point where I thought it was time to bury my firearms, it was actually time to pick them up..